In the construction of electrical equipment, efforts are increasingly being made to replace the hitherto used mechanical components, such as switches, push buttons and rotary potentiometers, by electronic components which are controlled by means of a digital data flow. When using digital technology, the disturbance-free transmission of the data, a simple indicator facility and the realizability of the necessary circuits in monolithic integrated semiconductor technology are ensured.
The invention relates to a circuit for an analog/digital converter with which the digital information corresponding to the electrical magnitude is converted into an associated and variable value of this electrical magnitude.
Such a circuit is necessary, for example, if a potentiometer necessary for the control of direct voltage levels is to be replaced by a digital/analog circuit. Such digital/analog converters are required for example in order to vary setting values on radios, televisions and tape recorders. In the case of a television, for example, the brightness, the color saturation and the loudness are adjusted by means of variable direct voltage levels.
Already several circuits have been proposed by which the conventional potentiometers can be replaced.
In the case of such a circuit, one piece of binary information corresponding to a direct voltage magnitude is written into a binary coded counter with weighted counter stages. A weighted resistance is connected after each counter stage, which all operate on a common summing resistance. The currents flowing in the weighted resistances according to the counter state are added up at the summing resistance so that a direct voltage corresponding to the counter state drops across this resistance. This circuit has the disadvantage that the accuracy of the resistance used determines the accuracy of the output voltage.
In the case of another known circuit, the information is converted over the desired voltage level into a binary word corresponding to this value. The individual places of this binary word are in each case written into a storage element associated with each place. The content of all the storage elements is interrogated through a resistance network, wherein the resistance value of each element is weighted corresponding to the appropriate place value of the cell associated with the resistance value. The currents flowing and weighted in the individual resistances according to the element content are then added in a common summing resistance connected thereafter and across which a direct voltage corresponding to the binary word thus drops.
This circuit also has the disadvantage that the accuracy of the output direct voltage is dependent on the accuracy of the resistance network. Moreover a circuit must be present in which the binary word corresponding to the desired voltage value is formed. For example a specified time is converted into a binary word corresponding to the time value, for producing the binary word.
In the case of a third known circuit, again a binary word is written into the storage cells associated with the places of the binary word. The binary word corresponds to the desired direct voltage. The binary word is now compared in a comparator with the contents of a continuously circulating counter. The place number of the counter must in this case correspond with the place number of the binary word. The counter thus changes its contents with a relatively high frequency, switches back to zero and begins to count again if the counter has reached the highest possible storable number. In the zero axis crossing of the counter, a bistable trigger stage connected to the comparator is so set that a voltage appears at its output. The bistable trigger stage is then reset by the comparator if the counter content coincides with the binary word. The pulse sequence leaving the output of the bistable trigger stage is fed to an integrator. Since the relationship of the time in which a voltage occurs at the bistable trigger stage output, to the time in which the output voltage of the bistable trigger stage is zero, corresponds to the binary word and thus to the desired direct voltage, the integrator emits this direct voltage value correctly and independently of the resistance values. However, a large electronic expenditure, by which makes the equipment very expensive, is required for the construction of this circuit.